The present invention is generally directed to operational amplifiers arranged for sensing the input voltages provided by a plurality of input voltage sources. The present invention is more particularly directed to such an operational amplifier which includes an internal switch means for selecting given ones of a plurality of input voltage sources.
Operational amplifiers find considerable use in modern electronic circuitry. Such amplifiers generally include a voltage sensing input, a reference input, and a plurality of amplification stages which provide an output voltage proportional to a sensed input voltage applied across the voltage sensing input and the reference input.
In many applications of operational amplifiers, it is advantageous for an operational amplifier to be used for sensing input voltages derived from a number of different sources. In the past, it has been customary to provide a switch disposed between the various input voltage sources and the voltage sensing input of an operational amplifier to selectively couple the input voltage sources, one at a time, to the voltage sensing input of the operational amplifier.
Operational amplifiers are often utilized in integrated circuits. In order to maintain processing compatibility, the switches, for selecting the input voltage sources, have customarily been solid state switches implemented by employing the same technology as the rest of the circuit in the integrated circuit.
While such arrangements have been generally successful, a problem has existed as a result of the fact that the solid state switches, implemented by a plurality of field-effect transistors, for example, have distributed or parasitic capacitance. The distributed capacitance provides a coupling path which couples noise from the integrated circuit power supply into the input of the operational amplifier. That noise is amplified by the various stages of the operational amplifier and appears at the operational amplifier output. In an open loop configuration, because the first amplification stage of an operational amplifier generally has the highest gain of all the stages and may have a gain of about one-hundred, the noise present at the operational amplifier input resulting from the input switch capacity is amplified by at least a factor of one-hundred. This can limit the utility of such an operational amplifier configuration.
By virtue of the present invention, these problems are overcome by employing the switching for the various input sources internally within the operational amplifier after the first amplification stage. As a result, the noise at the operational amplifier output is reduced by at least a factor equal to the gain of the first amplification stage of the operational amplifier.
It is therefore a general object of the present invention to provide a new and improved operational amplifier configuration which may be utilized for sensing input voltages provided by a plurality of input voltage sources.
It is a further object of the present invention to provide such an operational amplifier configuration wherein the switching between the various input voltage sources is performed after the first amplification stage of the operational amplifier to substantially reduce the resulting noise component at the first amplification stage output, and hence, at the operational amplifier output.
It is a still further object of the present invention to provide such an operational amplifier for use in an integrated circuit wherein the switching means for selecting the various input voltage sources may be compatible with the other circuits of the integrated circuit.